LTE (Long Term Evolution) and LTE Advanced which is an advanced type of LTE are next-generation systems providing communication which is faster and supports a larger capacity than the third-generation and the third-and-a-half generation cellular radio communication systems currently in operation. LTE and LTE Advanced are standardized by a standardization organization called 3GPP (3rd Generation Partnership Project).
In a downlink in the LTE systems (including LTE Advanced), a radio base station sends user data to a radio terminal by using a data transmission channel called PDSCH (Physical Downlink Shared Channel). Note that the downlink refers to communication in a direction from the radio base station to the radio terminal, whereas an uplink refers to communication from the radio terminal to the radio base station.
In LTE Advanced, provision of a heterogeneous network is under consideration. In a heterogeneous network, low-power base stations (so-called pico-cell base stations, femto-cell base stations, or relay nodes) are installed in a communication area of a high-power base station (a so-called macro-cell base station). A heterogeneous network is capable of distributing the load of the high-power base station to the low-power base stations.
However, since a radio terminal is generally connected to a radio base station which sends radio signals with the highest received power among multiple radio base stations, in the heterogeneous network the radio terminal may have a low chance of being connected to a low-power base station with low transmission power.
In consideration of such circumstances, there is proposed a method for expanding the coverage (communication area) of the low-power base station by performing control such that the radio terminal is connected to the low-power base station even when the received power from the low-power base station is not the highest (see, for example, Non-patent Document 1).